Examples of balancer constructions to reduce vibrations caused by motive engines are disclosed, for example, in JPA First Publication 1985-155033 and JPA Second Publication 1981-7536.
The balancer disclosed in the former patent application is constructed in such a way that a pair of balance weights is fixed to each of the balancer shafts arranged in parallel with the crank shaft. The secondary vibration by the engine is controlled by counter rotating counter weights of each of the balancers at a speed twice the rotational speed of the crank shaft.
In the case of single cylinder engines, the primary vibrational effects are quite serious, and it becomes necessary to control not only the secondary but the primary vibration caused by the engine.
If the primary vibration balancer is to be installed for a single cylinder engine according to this technique, it becomes necessary to provide a pair of primary balancer shafts separately from the secondary vibration balancer, and to rotate the secondary balancer shafts at the same rotational speed as the crank shaft and in the same direction. This type of design can lead to many problems, such as the necessity of providing extra space for the secondary balancer; complexity of the mechanisms for transmission of the rotational forces; thus leading to overall loss of freedom in design flexibility.
According to the disclosure in the second patent application, a pair of balancer shafts are located in parallel with the engine crank shaft, and fixing each of the balancer shafts with balancing weights; and disposing each of the balancer shafts within a given region of the coordinates of the crank shaft.
In this condition, the balance weights of the pair of balancer shafts are rotated at twice the rotational speed of the crank shaft, and in the counter direction to each other, to control the secondary vibration of the engine. By locating the balance weights within a certain region of the crank shaft, the vibrational moments created by the head and cylinder sections of the engine are controlled at the same time.
This technique has the following disadvantages.
Because the balancer shaft must be located in a designated region with respect to the crank shaft, the design freedom in engine layout becomes lost.
Furthermore, if the engine capacity should need changing, it becomes necessary to relocate the balancer shafts because the vibrational moments of the head and cylinder sections are also altered by changes in the engine capacity. Thus, the desirable generic nature of the engine becomes lost.
Additionally, the following general problems of the conventional engine balancing technology can be listed. In single cylinder engines, it is desirable to make the inertial momentum sufficiently high so a to produce low vibrations and a smooth running operation.
In general, in order to increase the inertial momentum of the crank shaft, it is desirable to:
1. Increase the outer diameter of a pair of balance weight sections attached to the crank shaft; PA1 2. Increase the thickness of the pair of balance weights in the axial direction of the crank shaft; PA1 3. Enlarge the size of the flywheel attached to the crank shaft. PA1 L1 is the inter-axial distance between the primary balancer shaft and the crank shaft; PA1 L2 is the inter-axial distance between the crank shaft and the secondary balancer shaft; and PA1 L3 is the inter-axial distance between the secondary balancer shaft and the tertiary balancer shaft.
However, if the approach 1 is adopted, it is necessary to increase the inter-axial dimensions between the crank shaft and the main shaft of the transmission to avoid mechanical interference of the crank shaft and the balance weight sections, thus leading to the undesirable result of increasing the size and weight of the engine. Also, an increase in the inertial momentum of the crank shaft can be achieved effectively by distributing the balancing mass away from the axial center of the crank shaft. However, by adopting the approach 2, because the outer diameter of the balance weight section is kept small, the increase in the inertial momentum is relatively low in comparison to the necessary weight increase of the crank shaft. The adoption of the approach 3 tends to lead to problems of vibrations caused by twisting or bending forces on the crank shaft, and stiffening of the shaft by increasing its size leads to the undesirable end result of an increasing weight of the crank shaft.